Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Exhaust gases produced by IC engines operating on fuels such as gasoline, diesel, liquefied petroleum gas (LPG), ethanol, natural gas and/or dual fuel variants include NOx gases, carbon monoxide (CO) and/or unburnt hydrocarbons which have to be neutralized before the exhaust gas is expelled into the environment. Aftertreatment systems employed for treating the exhaust gas produced by such IC engines operating under stoichiometric conditions often include a three way catalyst configured to efficiently decompose NOx gases, CO and unburnt hydrocarbons included in the exhaust gas. However, engines that operate lean cannot effectively decompose NOx gases with a three-way catalyst. A selective catalytic reduction (SCR) device, typically in conjunction with a reductant (e.g., urea-water solution (UWS)) injection system is generally used to efficiently decompose NOx emissions in a lean environment.
Three-way catalysts are effective when the engine is operated within a narrow band of air to fuel (air/fuel) ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen) conditions. Conversion efficiency falls very rapidly when the engine is operated outside of this band. Under lean engine operation, the exhaust gas contains excess oxygen, and the reduction of NOx is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, resulting in poor reduction of CO and unburned hydrocarbons and the formation of ammonia from NOx. From a fuel efficiency perspective, it is beneficial to operate the engine under lean conditions to minimize fuel consumption and maximize fuel efficiency. However, the limitation posed by the three way catalyst for operating within the narrow air to fuel ratio prevents this from being a feasible option. UWS injection, in conjunction with SCR technology, is used in some lean engine configurations. However, UWS SCR technology adds significant cost and control challenges.